The vital importance of the microfloral soil crust for ecology in arid and semiarid regions of the world has now been well established (e.g. www.soilcrust.org), and since arid and semi-arid climates comprise over one quarter of the land area of earth (more than any other climate type), the crust is eminently important for the global climate and ecology
The biogenic soil crust, known as biological soil crust or microphytic soil crust, being usually between 1 and 15 mm thick, and often covering up to 70% of the arid or semi-arid area, is inhabited predominantly by cyanobacteria, soil bacteria, algae, and lichens, which may be accompanied by mosses in more humid areas, the species composition varying from place to place [see e.g., Zaady, E. et al.: Soil Biol. Biochem. 32 (2000) 959-66]. Algae and cyanobacteria produce mucilaginous polysaccharides that seal the surface and form the crust.
The most important among the crust organisms are cyanobacteria, called also blue-green algae, organisms that have been present on the Earth for at least 3.5 billion of years, and have created the Earth's oxygen atmosphere. Cyanobacteria move through soil, and leave polysaccharide sheaths behind. The soil grains are glued together by said viscous polysaccharides secreted by cyanobacteria or green algae, and form a hard layer [Mazor G. et al.: FEMS Microbiology Ecology 21 (1996) 121-30]. Filamentous cyanobacteria, such as Microcoleus sp., are especially efficient in the crust formation in arid environments [Belnap J. and Gardner J. S.: Great Basin Naturalist 53 (1993) 40-7]. The soil particles are immobilized by being both glued to each other and entrapped in the web of fibers, wherein the cyanobacterial basis may be further strengthened by green algae, lichen species, or other plants, according to climate of the region [Zaady, E. et al.: Plant and Soil 190 (1997) 247-52]. From among lichens, which are algae or cyanobacteria symbiotically living with fungi, Colema may be named as a typical representative, important in microfloral crusts.
The crust layer may improve water hold-up by reducing its evaporation. The crust is involved in important ecological roles, including nitrogen fixation, preventing erosion, providing a first organic matter in the food chain, or affecting rain water runoff ratio [see, e.g., Zaady E. and Shachak M.: Amer. J. Botany 81 (1994) 109]. Although the crust is quite modest in some respects, not requiring more than 0.2 mm dew precipitation to keep photosynthesizing, it may be easily disturbed and damaged, e.g. by pushing, trampling by grazers, burning, or treating with herbicides, and its recovery may take from several to several dozens of years. An erosion, both by water and winds, are immediate results after the crust disturbance. In the United States alone, millions hectares have undergone some degree of wind erosion.
Several methods have been described for treating the areas with damaged soil crust. Small patches of heavily damaged soil may be recovered by scattering a sludge prepared by suspending 2-4 mm of upper layer of non-afflicted soil in water [St. Clair L. L. et al.: Reclam. Reveget. Res. 4 (1986) 261-9; Metting B.: Biological surface features of semiarid land and deserts. In Skujins, J., (Ed.), Semiarid lands and deserts, Soil resource and reclamation. Marcel Decker Inc., New York. (1991) pp. 257-93]. Transferring large volumes of soil from one place to another is a drawback of the method. Further, the method is invasive, damaging the site from which the soil is taken, especially in view of an estimation that about 75% photosynthetic biomass is produced in the top 3 mm of soil. Another method comprises spraying the soil with an algae suspension, prepared from laboratory cultures [e.g., Johansen J. R: Journal of Phycology 29 (1993) 140-7]. Disadvantages of the method comprise high costs of the cultures, and a danger of spreading species non-native to the treated environment. A still other method includes adding algal polysaccharides into the soil. However, the soil organisms feed on the saccharides, leading to their quick disappearance. U.S. Pat. No. 3,969,844 describes a method of conditioning soil by inoculating said soil with flocculant-producing algae, however all nutrients necessary for the growth of said algae must be continually maintained in said soil. U.S. Pat. No. 4,774,186 discloses the production of an algae powder, suspendible in water and sprayable on soil to improve its aggregation. U.S. Pat. No. 5,441,877 discloses a method for the vegetalization of bare terrain comprising applying a substrate consisting of an organic matter, synthetic polymer, and a clay inoculated with cyanobacteria and mosses. U.S. Pat. No. 6,228,136 discloses a method of aggregating soil, comprising inoculating a substrate material, such as a woven fabric, with cyanobacteria, and disseminating the substrate pieces in the soil. The latter method comprises identifying or isolating naturally occurring species of cyanobacteria in the intended area.
Finally, a method has been described, that comprises adding to the soil artificial stabilizers which glue soil particles together [Rubio H. O. et al.: J. Range Management 45 (1992) 296-300]. However, said method does not support the appearance of naturally occurring, crust-forming organisms. The appearance of these organisms depends on immission of the reproduction-ensuring material (further called propagules), originating in the undisturbed areas, that passively moves to disturbed areas and may germinate to establish a new crust, and a passive drift like that may be very slow.
It is therefore an object of the invention to provide a new method for enhancing the soil crust formation, comprising accelerating the colonization by microorganisms native to the treated environment.
It is still another object of the invention to provide a method for enhancing the soil crust formation without the necessity to identify or isolate the species of the involved microorganisms.
It is yet another object of the invention to provide a method for enhancing the soil crust formation in a target region, said method being non-invasive, avoiding damaging the soil crust in another region, and avoiding disseminating any biological or chemical material not endemic to the target region.
The new method should accelerate the crust formation, and should support rehabilitation efforts in arid or semiarid areas, or in a disturbed region, i.e. in a region that underwent any crust disturbance. This invention also aims at providing a method for enhancing the formation of the soil crust in an area that is contaminated or potentially contaminated with environmentally undesired materials, since the absence of the soil crust layers may be the main reason for spreading contaminations from dumps, waste disposal sites, or otherwise littered or contaminated areas. It is therefore a further object of the invention to slow down or to prevent said spreading.
Many methods have been developed for containing hazardous materials, and for reducing their spreading into the environment from dump sites, including concrete layers, mineral layers, plastic layers, etc., wherein said site is separated either from adjacent soil or also from adjacent air by various interfaces. WO 90/14222 describes a method of preventing migration of hazardous wastes to the environment, comprising a leakproof composite layer consisting of a mineral layer and a flexible sheet. U.S. Pat. No. 5,090,843 discloses a seal layer covering a waste disposal site, comprising a layer of mineral, such as calcium carbonate, and a layer of soil above said mineral layer. The seal layer is intended to prevent the migration of water into or out of a waste material. The materials used for isolating the dump sites from the adjacent environment have limited stabilities, and it is desirable that the soil adjacent to said dump site, as well as the soil eventually used in isolating said dump site, have reduced permeability to water, particularly to rain water, that may be the main factor in spreading hazardous materials from the dump sites to the environment.
It is therefore a further object of the invention to stabilize the surface of the soil in dump sites, or the soil in the vicinity of dump sites, in order to reduce or prevent a leakage of contaminants from said site into the environment. The term leakage or spreading, as used herein, comprises transmission of environmentally undesired materials from contaminated areas to uncontaminated or less contaminated areas by surface water, by groundwater, or by wind.
It is a still further object of the invention to stabilize the soil surface in contaminated areas, or near contaminated areas, using organisms naturally occurring in the environment of the treated area.
It is also an object of the invention to stabilize the soil surface in contaminated areas, or near contaminated areas, wherein the contamination originates from the waste disposal site, or from unwanted leakage.
It is also another object of the invention to enhance the crust formation in the soil that separates the predominantly contaminated area from the predominantly unaffected area.
Other objects and advantages of present invention will appear as description proceeds.